This invention relates to hollow fiber membranes for the degassing of chemical liquids and, more particularly, to composite hollow fiber membranes for the degassing of chemical liquids for use in semiconductor fabrication processes (e.g., photoresist solutions, developing solutions, and chemical liquids for use in a spin-on-glass process), inks for ink-jet printers, liquid crystals and organic solvents.
With respect to high-purity water for use in semiconductor fabrication, water drained from boilers, and the water supply lines of buildings, raw water containing dissolved oxygen in a saturated state tends to be responsible for oxidation. Accordingly, Japanese Patent Laid-Open No. 169303/""91 discloses a technique for reducing the dissolved oxygen concentration by removing dissolved oxygen from such raw water with the aid of a hollow fiber membrane. In this patent, there is used a three-layer composite hollow fiber membrane in which a homogeneous film consisting of a thin silicone rubber film, silicone-polycarbonate copolymer, poly(4-methylpentene-1), perfluoroalkyl polymer or segmented polyurethane is interposed between porous films.
Water-containing chemical liquids include semiconductor developing solutions, inks for ink-jet printers, alcohol-water mixtures and the like. In the case of such chemical liquids, they are sometimes used at a temperature lower than 30xc2x0 C. so that their gas solubilities may be reduced to the utmost. The reasons why this method of use is employed are that, since water tends to have lower gas solubilities than organic solvent over a wide temperature range, the use of a water-based solution is suitable for the purpose of reducing dissolved gas concentrations, and that the gas solubilities become lower as the temperature of the solution is reduced.
However, when the method described in the aforementioned patent is applied to water-containing chemical liquids, the following problems arise. In the case of a hollow fiber membrane in which a thin silicone rubber film, silicone-polycarbonate copolymer, perfluoroalkyl polymer or segmented polyurethane is used as the homogeneous film, it has not only high oxygen and nitrogen permeation fluxes, but also a high water vapor permeation flux. Consequently, during the course of degassing, condensation causes waterdrops to be formed on the opposite surface of the membrane. This is the same situation as the leakage of raw water to the opposide surface. Moreover, the membrane swells severely with alcohols, ethers, ketones and esters contained in chemical liquids, so that the thin film may be broken or a large amount of solvent vapor may be discharged to the secondary side (or evacuated side) of the membrane. In the case of a hollow fiber membrane using poly(4-methylpentene-1), this membrane material has its glass transition region in the vicinity of 30xc2x0 C. and hence becomes brittle in water having a temperature lower than 30xc2x0 C. Consequently, the membrane tends to be broken by the action of external pressure and cause the leakage of raw water to the opposite surface.
In semiconductor fabrication processes, defects such as processing spots may be produced owing to gas bubbles introduced into the supplied chemical liquids. For example, in the lithographic process in which a thin film overlying a semiconductor wafer is patterned by coating with a photoresist solution, exposure to light through a pattern-bearing mask, development and etching, troubles such as poor patterning due to processing spots may be developed when the semiconductor wafer is spin-coated with a photoresist solution or developing solution having gas bubbles introduced thereinto. Moreover, when gas bubbles are introduced into the washing fluid used in the washing operations of the lithographic process, washing spots may be produced.
The cause for the introduction of gas bubbles is believed to be as follows. A chemical liquid is delivered to a discharge nozzle with the aid of nitrogen gas. When the chemical liquid is discharged from the nozzle, the pressure applied to the chemical liquid is returned to atmospheric pressure. As a result, the dissolved gas present therein becomes supersaturated and this supersaturated fraction forms gas bubbles. The formation of gas bubbles can be minimized by reducing the dissolved gas concentration in the chemical liquid delivery step according to a technique such as membrane degassing.
As techniques for removing dissolved gases from chemical liquids by use of a membrane, the following methods have been known.
(1) a method for removing dissolved nitrogen from a chemical liquid by use of a porous hollow fiber membrane having intercommunicating pores from the inner surface to the outer surface (Japanese Patent Laid-Open Nos. 243306/""96, 94447/""97, 7936/""97, 199607/""89 and 7915/""89; Japanese Patent Publication Nos. 57478/""93 and 45282/""93; and the like).
(2) a method for removing dissolved nitrogen from a chemical liquid by use of a heterogeneous hollow fiber membrane in which a homogeneous thin film layer (i.e., a thin film layer free of intercommunicating pores) is disposed on the surface and supported by a porous substrate layer formed from the same polymer as the homogeneous thin film layer (Japanese Patent Laid-Open Nos. 94447/""97, 187629/""97 and 278897/""94; and the like).
(3) a method for removing dissolved nitrogen from a chemical liquid by use of a non-porous (homogeneous) tubular membrane prepared by forming a tetrafluoroethylene resin having excellent solvent resistance into tubes (Japanese Patent Laid-Open Nos. 153675/""97, 243306/""97, 7936/""97, 267149/""93, 31804/""95, 57008/""97 and 124875/""96; Japanese Utility Model Laid-Open No. 9160/""90; and the like).
(4) a method for removing dissolved nitrogen from a chemical liquid by use of a two-layer composite hollow fiber membrane comprising a homogeneous thin film laminated onto a porous substrate layer (Japanese Utility Model Laid-Open No. 91601/""90; and Japanese Patent Laid-Open Nos. 243306/""97, 94447/""97 and 68007/""89).
(5) a method for removing dissolved nitrogen from a chemical liquid by use of a three-layer composite hollow fiber membrane prepared by interposing a non-porous film comprising a fluororesin between porous substrate layers comprising a fluororesin, and adhesive-bonding the layers with together to form an integral structure (Japanese Patent Laid-Open No. 63007/""89).
However, in the method (1), successful degassing cannot be achieved if the membrane material is highly wettable by the chemical liquid. Specifically, when the liquid being treated is fed to the primary side of the membrane, the liquid being treated penetrates into the pores and leaks out to the secondary side (i.e., the opposite side) of the membrane. This phenomenon is noticeable especially when the chemical liquid comprises a chemical liquid for use with semiconductors or an ink for ink-jet printers.
In the method (2), it is difficult to disturb the crystalline orientation of the homogeneous thin film layer perfectly in the spinning step, so that an ordered structure based on crystalline orientation is created to some extent. As a result, intercommunicating pores tend to be formed in the homogeneous thin film layer during the course of the drawing step for forming a porous layer. Moreover, during handling of the prepared membrane, mechanical rubbing tends to produce pinholes in the homogeneous thin film layer. With this membrane, therefore, successful degassing cannot be achieved because the liquid being treated penetrates into the pores of the porous layer and then leaks out through the pores or pinholes of the homogeneous thin film layer. Such leakage of the chemical liquid is noticeable especially when the chemical liquid comprises a chemical liquid for use with semiconductors or an ink for ink-jet printers.
In the method (3), not only the membrane material has a low nitrogen permeability coefficient, but also the tubes used for degassing purposes have a large membrane thickness. Consequently, the nitrogen permeation flux is low [e.g., nitrogen permeation flux=0.5xc3x9710xe2x88x9211 cm3/(cm2xc2x7Pa xc2x7sec)], so that degassing causes the dissolved gas concentration to be reduced only to about 90% of its saturated concentration. This degassed level is insufficient for practical purposes.
In the method (4), both the homogeneous thin film material and the porous substrate material have solvent resistance and are hence chemically inert, so that it is difficult to bond both layers with an adhesive. Accordingly, both layers are integrally bonded by fusion, but the fusing step tends to produce pinholes in the homogeneous thin film. When pinholes are produced, satisfactory degassing cannot be achieve because the liquid being treated leaks out through the pinholes.
In the method (5), the thickness of the non-porous film is enlarged by the thicknesses of the adhesive layers, resulting in an increase in resistance to gas permeation. Consequently, the fluxes of the gases passing through the thin film are substantially reduced and, therefore, degassing capability tends to be insufficient for practical purposes.
With regard to the liquid crystal sealing process, techniques for removing dissolved gases or gas bubbles present in a liquid crystal by means of a vacuum pump are disclosed in Japanese Patent Laid-Open Nos. 170932/""98 and 218921/""95. To this process, the same techniques as described above in connection-with the degassing of chemical liquids for use in semiconductor fabrication processes are also applicable, but still involve the same problems.
In ink-jet printers having a piezoelectric element head, the piezoelectric element of the head repeats pressurization and depressurization many times during ink discharge. It is known that, during this process, dissolved gases (e.g. dissolve oxygen and nitrogen) present in the ink contained in the head grow to gas bubbles, which tend to stay in the piezoelectric element head. These gas bubbles may be discharged together with the ink to cause print dot losses. On the other hand, in ink-jet printers having a thermal head, it is known that, as a result of thermal cycles for heating or cooling the ink rapidly, dissolved gases present in the ink grow to gas bubbles during head operation. Since these gas bubbles tend to stay in the thermal head, they may be discharged together with the ink to cause print dot losses.
As a technique for removing dissolved gases from an ink by means of a membrane, Japanese Patent Publication No. 37173/""95 discloses a method for removing dissolved gases present in an ink through a plain-film partition positioned in the ink discharge head unit so as to separate an evacuated space from an ink-filled space. However, since the head unit of an ink-jet printer must be rapidly operated, there is a limit to the area of the membrane which can be mounted in the head. Since the membrane materials described therein have oxygen and nitrogen permeation fluxes of as low as about 1 to 3 10xe2x88x9210 cm3/(cm2xc2x7Paxc2x7sec), it is difficult to remove dissolved gases to a sufficient extent.
Moreover, Japanese Patent Laid-Open No. 17712/""93 discloses a method for removing dissolved gases present in an ink selectively through a hollow fiber membrane by feeding a raw ink to the inside of the membrane and evacuating the outside of the membrane. However, the tetrafluoroethylene tubes used in the examples of this patent have a very small membrane thickness of 1 to 2 xcexcm. When a raw ink is made to flow on the inside of the membrane, the flow of the ink causes the membrane to expand outwardly. Since the membrane thickness is very small relative to this force and hence has insufficient mechanical strength, there is a strong possibility that the membrane may be broken to cause leakage of the liquid being treated.
An object of the present invention is to provide a degassing membrane which has sufficiently high oxygen and nitrogen permeation fluxes and a low water vapor permeation flux, and can be used to degas chemical liquids without any liquid leakage.
That is, the present invention provides a composite hollow fiber membrane having a composite structure consisting of a homogeneous thin film interposed between porous substrate layers wherein the ratio of the oxygen permeation flux to the nitrogen permeation flux of the membrane is not less than 1.1 and wherein, after the membrane has been immersed in chemical liquids according to JIS K 7114, the percentage change of the ratio is within xc2x110%.
It is preferable that, after this composite hollow fiber membrane has been immersed in chemical liquids according to JIS K 7114, the percentage change in weight of the membrane is in the range of 0 to +30%. It is also preferable that this hollow fiber membrane has a nitrogen permeation flux of not less than 0.5xc3x9710xe2x88x929 cm3/(cm2xc2x7Paxc2x7sec) and an oxygen permeation flux of not less than 0.6xc3x9710xe2x88x929 cm3/(cm2xc2x7Paxc2x7sec).
In the present invention, the chemical liquid immersion test according to JIS K 7114 is carried out by using five types of chemical liquids including isopropyl alcohol, a semiconductor developing solution, a spin-on-glass solution, an ink for ink-jet printers, and a liquid crystal. For this testing purpose, a 2 wt % aqueous solution of tetramethylammonium hydroxide is used as the semiconductor developing solution, a mixture composed of 70% by weight of isopropyl alcohol, 2% by weight of tetraethoxysilane, and 28% by weight of water as the spin-on-glass solution, a dye-based ink containing a solvent composed of 80% by weight of water, 5% by weight of ethylene glycol, and 15% by weight of isopropyl alcohol as the ink for ink-jet printers, and cholesterin chloride cholesterin nonanoate as the liquid crystal.